Mounting Stepper Motor to Molded Worm Gears

The design goal was to mount the motor loosely to dampen the sound transmitted into the scope. The motor shaft should not have to take any axial load from the worm and the worm should not move length wise. This design accomplishes these goals and can be built with simple hand tools. The parts are easy to get. I got all of mine at the local hardware store.

Parts for one axis

· Main support – One the left in the picture. ¾ inch plywood aluminum edging about 1 foot long. It has already been modified in the picture.

· Motor isolation hardware - 2 rubber grommets and an inch of ¼ inch model airplane fuel line hose and 2 #6 nylon screws and nuts

· Motor mounting bracket – Not shown. 1 inch by 1inch aluminum angle. Sized to fit motor. Mine was about 2 inches long.

· Bearings - roller blade bearings. 5/16 inch inner diameter, .865 inch outside diameter, .275 inch thick.

· At least 2 5/16 – 18 nuts. To mount the worm to the main support.Worm - 5/16 -18 threaded rod

Construction

The key features of this design are the way the bearings are attached to the worm and the main support. At least one bearing must be fixed to the worm trapped between two nuts. It must not be able to move back and forth on the worm shaft.

Bearing secured by slots in main support

The bearings on the rod assembly are then set into slots cut into the sides of the aluminum channel. The slot must be the width of the bearing. The bearings must be a tight fit in the slots and not be able to move back and forth toward the motor. The slot does not need to be cut all the way to the bottom of the channel. Leave the bottom of the slot at least ¼ inch above the bottom of the channel to keep the Main support from bending under load.

Main support made from ¾ “ plywood edge trim

The main support holds the bearing and the motor and has a pivot for mounting the assembly. Space the bearings far enough apart so that they will clear the main gear the worm will mate with. On my scope that meant they must be about 6 1/2 inches apart. Cut the slots in the channel to accept the bearings. You can also cut away parts of the channel the might interfere with the rest of the system. I also drilled large holes in the bottom to let dirt fall through. Drill and tap two small holes at the motor end of the channel to attach the motor bracket. I used 4-40 screws. You could nuts instead of tapping holes.

Motor mounting bracket made from 1x1 inch angle stock.

Drill holes to match the motor hole spacing on one side and small holes to match the ones on the main support bracked on the other side. Insert rubber groumets into the motor mount holes.

Stepper sound and vibration isolation

Cut 2 pieces 1/4 inch long of the model fuel line hose and put them in the mounting holes of the motor. Attach the motor to the mounting bracket using #6 nylon screws. The screws will thread into the hose but you can also add nuts if you with. I left one of the screws partially out in the picture to make it show up better.

Assembly

Attach the mounting bracket with the motor attached to the main support using the 2 #4 screws. Attach the worm to the motor using a piece of 1/4 inch automotive fuel line hose. This will also isolate much of the motor noise from the worm and creates a flexible coupling to the worm so that motor alignment is not critical. Space the bearings on the worm and insert the assembly into the main support slots. To keep the bearings tight in the slots I used tie wraps.

Decide where and how you want to mount the assemblies then drill a hole through the side of the channel for the combination pivot and mounting hole. This hole must be very close to the bottom of the channel so that the mounting bolt does not interfere with the worm. The hole must also be perpendicular to the channel in both X and Y directions. A drill press or drill guide is a great help here. I used 1/4 inch diameter bolt for the Altitude motor and # 10 for the AZ motor. I mounted a small spring under the end of the main support to help push the worm into the gear.

Completed units attached to scope base.

Gravity also helps because the Altitude motor is heavier than the worm. For the AZ I mounted a torsion spring on the pivot to push the worm into the gear. Later I also added an additional spring at the other end to put more force on the gear.